WASHINGTON, July 20, 2000 — Scientists call it the “Great Dying,” a period of unrivaled carnage in our planet’s history. About 251 million years ago, Earth’s greatest mass extinction event wiped out more than 90 percent of all marine plants and animals and up to 70 percent of all land species. The extinction marked the end of an era, closing the book on the first great pulse of life on Earth and ushering in the age of reptiles. But the Great Dying is also a great mystery, maybe the greatest murder mystery ever, because the “murderer” is still at large.

Like police investigators, scientists hope to zero on the extinction’s cause by reconstructing the details of the scene of the crime, such as the timing and pattern of the catastrophic species turnover. Now, research published in Friday’s issue of the journal Science suggests that this massive die-off may have taken place within a mere 500,000 years or less — a geologic snap of the fingers.

Other tantalizing clues reported by the Science researchers, including tiny metallic spheres gleaned from extinction-age rock layers, could help whittle down the lineup of lethal suspects in this case. Rising and falling sea levels, massive volcanic explosions, glaciers, acid rain, global warming and cooling, and even an extraterrestrial impact like the one that killed off the dinosaurs have all been fingered as the culprits behind the crisis 250 million years ago.

The crime scene
The Great Dying is also known as the Permian-Triassic (P-T) extinction, since it falls at the end of the Permian geologic period and the beginning of the Triassic period. The Science researchers — including Jin Yugan of the Nanjing Institute of Geology and Paleontology in China, and Douglas Erwin of the National Museum of Natural History, Smithsonian Institution in Washington, D.C. — study the P-T extinction at the site of Meishan in southern China, where a series of rock layers spans the boundary between the two periods.

Mass extinctions can happen suddenly, gradually, or in a series of steps, each leaving a different pattern in the fossil record. In the past, scientists thought that the Meishan fossil record resembled a stepped pattern, recording a series of smaller extinctions spread out over a few million years around the P-T boundary.

“A number of different people have argued for at least three different extinction horizons at Meishan,” says Erwin, “so we decided to see if this pattern was real, or something influenced by the preservation of the fossils at the site.”

To test the pattern, the researchers traced the disappearance of 333 marine fossil species in the rock layers above and below the Meishan P-T boundary. These species included a wide range of invertebrates with shells similar to modern clams and snails, coral reef builders, squid-like creatures, sea lilies, fish, algae, and one of Earth’s animal pioneers, the hard-shelled invertebrates called trilobites.

At Meishan, fossil-rich rock layers are sandwiched between layers of volcanic ash that are already well-dated by isotopic analysis. With the help of these ash layers, the researchers created a series of dates spanning the entire P-T section, allowing them to pinpoint the last time each of the 333 species in their study appeared in the fossil record at the site.

Their analysis suggests that most of the species in their study vanished from the fossil record 251.4 million years ago, in rock layers right around the P-T boundary. Within these layers, the species extinction rate skyrocketed to 94 percent, up from a steady 33 percent in the rock layers below the P-T boundary. This dramatic increase in the extinction rate happened suddenly, occurring in less than 500,000 years.

According to Jin, these numbers convinced the researchers that they were looking at a single and sudden mass extinction event at Meishan, rather than several smaller extinctions clustered around the P-T boundary. The extinction peak in the fossil record coincides with a significant shift in carbon isotope ratios, which may indicate a devastating population crash of marine life. Most species probably went extinct 251.4 million years ago, followed by the disappearance of a few lingering survivors over the next one million years.

Usual (and unusual) suspects
The researchers say that this new information about how and when the P-T mass extinction took place should help scientists refocus their efforts on clues pointing towards the extinction’s cause.

One intriguing clue is scattered throughout the Meishan layers-superheated debris particles called microspherules. In southern China, the Science researchers report, P-T boundary rock layers contain 100 to 1,000 times more of these tiny metallic and glassy spheres than other rock layers. This discovery mirrors similar finds in the rock layers that mark the later extinction of the dinosaurs.

Do the P-T microspherules come from a cosmic impact like the one that ended the dinosaurs’ days on Earth? Little evidence supports such an impact, says Erwin, although the possibility can’t be completely ruled out. He notes that the microspherules may have their origin in another timely cataclysmic event-the massive eruption of the Siberian Traps.

The Siberian Traps are huge flood basalts that erupted at the time of the P-T extinction, quickly pouring forth a volume of molten lava big enough to blanket modern Europe.

The Siberian volcanism could have spewed out clouds of carbon dioxide or sulfur dioxide, provoking cycles of global warming and cooling, acid rain, and sea level fluctuations that all came together in the P-T crisis.

Even if this massive eruption, or a cosmic impact, were the major triggers of the P-T crisis, Erwin says, they were probably only part of a web of factors that eventually culminated in the greatest-ever extinction.

Case closed?While researchers continue to investigate the mystery of the Great Dying, Erwin’s interest lies in what happened afterward — call it the Great Rebirth. He would like to use the clues provided by the P-T extinction to examine how life rebuilt itself after such a tremendous setback, examining the nature of the plants and animals that survived and the timing and patterns of their rebound. The P-T extinction’s lessons of recovery, Erwin notes, may be more important to understanding life’s history than the crisis itself.

“The Permian-Triassic extinction was the most fundamental change in life from the origin of life up until today,” Erwin says. “What you see in a tidepool today is based on what lived and died 251 million years ago.”